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Rear Main Seals
Pontiac V8 Rear Main Seals
Stop the Drip — Rear Main Seals for Pontiac V8s
The rear main seal is the one leak everybody notices, because it drips right where the trans bolts up. Since asbestos came out of the factory rope seals, leakage on older Pontiacs became routine — which is why modern Viton seals have largely replaced them.
Butler stocks Pontiac rear main seals by main journal size: 3.00" mains for 301, 326, 350, 389 and 400, and 3.25" mains for 421, 428 and 455. Choices include BOP two-piece single lip Viton, BOP one-piece dual lip Viton, SCE AccuSeal Pro Viton, and Best Gasket braided Teflon/graphite rope seals.
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More About Rear Main Seals
1. First, get the main journal size right
Pontiac used two rear main journal sizes, and nothing else about the seal matters until you know which one you have.
| 3.00" main | 1959-79 326, 350, 389 and 400, plus the 1979-81 265 and 301. The early 347 and 370 can use this seal when trimmed to fit. |
| 3.25" main | 421, 428 and 455. |
| Not Pontiac | Butler also lists an Oldsmobile 403 Viton seal. It is a different engine family — do not order it for a Pontiac. |
2. Two-piece or one-piece, and the question that decides it
Both styles are available in Viton for both journal sizes, but they are not interchangeable in the middle of a job.
| Two-piece single lip | the traditional style, available from BOP and from SCE as an AccuSeal Pro Viton seal. This is the one you want if the crankshaft is staying in the block — the upper half rolls into the groove behind the crank. |
| One-piece dual lip | BOP's design relocates the parting line to greatly reduce the risk of leaking, uses a more pliable construction that works better in blocks with seal groove geometry issues, and adds a second lip for twice the protection against leakage and better vacuum numbers. |
| The catch | the one-piece seal cannot be installed with the crankshaft in the block. If the engine is together and you are chasing a leak from underneath, you need the two-piece seal. This is the most common reason a rear main seal comes back to Butler as a return. |
3. Rope seals and installation basics
There is still a place for a rope seal, and there are a few habits that keep any of these from weeping.
| Braided rope | Best Gasket braided Teflon/graphite rope seals are stocked for 326-400 and for 421-455 — the correct choice for a period-correct rebuild or a block whose groove suits a rope seal. |
| Dry fit first | set the seal in the groove before final assembly. If it bulges, trim it with a sharp knife rather than forcing it. |
| Sealer, sparingly | a thin film at the seal ends is normal practice. Do not coat the back of the seal or pack the groove. |
| Anti-rotation holes | the BOP seals depend on the anti-rotation holes being handled correctly. Read the instruction sheet that comes with the seal before it goes in — it is a five-minute read against an engine-out repair. |
Viton has become the default here for a reason: it holds up to heat and to modern oils. Order by journal size, then decide two-piece or one-piece based on whether the crank is coming out.
Not sure what your Pontiac needs?
Call our tech team at 866-762-7527. Tell us your engine, heads, car, and power goal, and we will put the parts together as one combination that works.